Passivation of ceramic piezoelectric ink jet print heads

ABSTRACT

This invention relates to improvements in or relating to ceramic piezoelectric ink jet print heads of the kind having an ink channel for connection to an ink ejection nozzle and to a reservoir for the ink, and a piezoelectric wall actuator which forms part of the channel and is displaceable in response to a voltage pulse thereby generating a pulse in liquid ink in the channel due to a change of pressure therein which causes ejection of a liquid droplet from the channel. Such print heads are referred to hereafter as piezoelectric ceramic ink jet print heads.

FIELD OF THE INVENTION

This invention relates to improvements in or relating to ceramicpiezoelectric ink jet print heads of the kind having an ink channel forconnection to an ink ejection nozzle and to a reservoir for the ink, anda piezoelectric wall actuator which forms part of the channel and isdisplaceable in response to a voltage pulse thereby generating a pulsein liquid ink in the channel due to a change of pressure therein whichcauses ejection of a liquid droplet from the channel. Such print headsare referred to hereafter as piezoelectric ceramic ink jet print heads.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of print heads as described, for example, in EP-A-277703,EP-A-278590 and EP-A-364136 are shown in FIGS. 1, 2, 3 and 4.

FIGS. 1 and 2 are different sectional views of the same ink jetprinthead, and

FIG. 3 is a view similar to that of FIG. 1 showing another form ofprinthead.

FIG. 4 is a greatly enlarged view of an ink channel defined by walls insuch a print head.

BACKGROUND OF THE INVENTION

One form of ink jet printhead 10 comprises a multiplicity of parallelink channels 12 forming an array in which the channels are mutuallyspaced in an array direction perpendicular to the length of thechannels. The channels are formed at a density of two or more channelsper mm. in a sheet 14 of piezoelectric material, suitably PZT, poled inthe direction of arrows 15 and are defined each by side walls 16 and abottom surface 18, the thickness of the PZT being greater than thechannel depth. The side walls 16 are generally at an angle of no morethan 10° from the normal to the bottom wall. The channels 12 are opentopped and in the printhead are closed by a top sheet 20 of insulatingmaterial which is thermally matched to the sheet 14 and is disposedparallel to the surfaces 18 and bonded by a bonding layer 21 to the tops22 of the walls 16. The channels 12 on their side wall surfaces arelined with a metallised electrode layer 34. It will be apparenttherefore that when a potential difference of similar magnitude butopposite sign is applied to the electrodes on opposite faces of each oftwo adjacent walls 16, the walls will be subject to electric fields inopposite senses normal to the poling direction 15. The walls are inconsequence deflected in shear mode.

Referring now to FIG. 2, the channels 12 therein are provided on facingwalls 16 thereof with metallised electrodes 34 which extend from theedges of the tops 16 of the walls down the walls to a location wellshort of the bottom surface 18 of the channels. There is an optimummetallisation depth which gives maximum wall displacement at about themid-height of the walls depending on the distribution of wall rigidity.In this form the walls are of the so-called cantilever type.

In FIG. 2, it will be seen that the channels 12 comprise a forward part36 of uniform depth which is closed at its forward end by a nozzle plate38 having formed therein a nozzle 40 from which droplets of ink in thechannel are expelled by activation of the facing actuator walls 16 ofthe channel. The channel 12 rearwardly of the forward part 36 also has apart 42 of lesser depth extending from the tops 22 of the walls 16 thanthe forward part 36. The metallised plating 34 which is on opposedsurfaces of the walls 16 occupies a depth approximately one half that ofthe channel side walls but greater than the depth of the channel part 42so that when plating takes place the side walls 16 and bottom surface 18of the channel part 42 are fully covered whilst the side walls in theforward part 36 of the channel are covered to approximately one half thechannel depth in that part. One suitable electrode metal used is analloy of nickel and chromium, i.e. nichrome. Alternatively, aluminiumprovides a high conductivity electrode and the metal track in the part42 is suitable for applying a wire bond connection. Aluminium inparticular requires to be coated with a layer of passivation to inhibitelectrolysis and bubble formation or corrosion which could occur if theelectrode is in direct contact with the ink.

It will be noted that a droplet liquid manifold 46 is formed in the topsheet 20 transversely to the parallel channels 12 which communicateswith each of the channels 12 and with a duct 48 which leads to a dropletliquid supply (not shown).

In the arrangement shown in FIG. 3, wherein elements common with theembodiment of FIGS. 1 and 2 are identified by the same referencenumerals as in FIGS. 1 and 2, a sheet 14 is employed therein havingupper and lower regions poled in opposite senses as indicated by thearrows 15. A sheet 50' of glass or other insulating material is employedas a stiffening means for the sheet 14 of piezo-electric material. Theelectrodes 34 are deposited so as to cover the facing channel side wallsfrom the tops thereof down to a short distance from the bottoms of thechannels so that a region of each side wall extending from the top ofthe channel and poled in one sense and a substantial part of a lowerregion of the side wall poled in the reverse sense are covered by therelevant electrode. Thus, it will be appreciated that the arrangementdescribed operates to deflect the channel side walls into chevron form.Other forms of ink jet printhead having an array of ink channelsseparated by piezoelectric wall actuators described in the art are alsosuitable for the application of the process of this invention.

The invention is concerned with passivation of the walls of thechannels; that is, the deposition of a protective layer on the walls bycoating. The purpose of the passivation is to provide a coating actingas an electron or ion or ink barrier and therefore to protect thechannel walls from attack by the ink and/or to protect the ink from thechannel walls. Protection of the channel walls from the ink isparticularly desirable where the ink is aqueous or otherwiseelectrically conductive.

Where--as is the case in the example given above--the channel includesopposed walls comprising piezoelectric ceramic material and is providedwith electrodes for connection to voltage pulse generating means,passivation is particularly desirable to protect the electrodes from theink and also to insulate the ink from the electrodes, and moreparticularly the fields generated by the electrodes, especially wherethe ink is a dispersion. In one embodiment of this form of ink jet printhead, the channels are formed with opposed side walls and a bottom wallall of piezoelectric ceramic material, e.g. by cutting or machining anopen channel from a block of the material, and a top wall which closesthe channel. In this embodiment, in general the side walls and bottomwall are passivated.

IBM Technical Disclosure Bulletin, Vol. 23, No. 6, November 1980, page2520 discloses a method for passivation of an ink jet silicon nozzleplate whereby a first overcoat of thermal SiO₂ is applied to a siliconsubstrate followed by a second overcoat of glow discharge siliconcarbon. Formation of the first overcoat generally entails substratetemperatures of the order of 900° C.

EP-A-0 221 724 discloses an ink jet printer nozzle having a substrate ofsilicon or glass and a coating resistant to corrosion by aqueous andnon-aqueous inks. The coating comprises respective layers of siliconnitride, silicon nitride with aluminium nitride, and aluminium nitride.Sputtering, Chemical Vapour Deposition (CVD) and evaporation are givenas suitable techniques for forming the coating. Typical substratetemperatures are given as 700°-800° C. and, as described, ion-assisteddeposition is a line-of-sight coating process.

U.S. Pat. No. 4,678,680 discloses the use of an ion beam implantingdevice to implant ions in the aperture plate of an ink jet printer ofthe continuous stream type, thereby improving the corrosion resistanceof the aperture plate.

IBM Technical Disclosure Bulletin, Vol. 22, No. 8, January 1979, page3117 discloses a method of depositing a coating material such astitanium on to the bore of a nozzle using ion plating. This methodrelies on resputtering of that coating material initially deposited nearthe mouth of the bore of the nozzle so as to achieve coating furtherinside the bore.

Achieving the substantially continuous coating of the channel walls of aceramic piezoelectric printhead that is required for effectivepassivation gives rise to particular problems, however. One problem isthat certain areas of especially the lower parts of the side walls of achannel cannot readily be coated by procedures which requireline-of-sight between the coating source and the surface to be coatedbecause, when the source is appropriately located relative to thechannel for deposition of a layer on a side wall, these lower parts willbe in the shadow of the upper part of the opposite wall. Moreover, thisproblem increases with the depth of the channel relative to its width(referred to hereafter as the "aspect ratio" of the channel).

Another problem particular to this type of ink jet printhead is causedby the granular structure of the piezoelectric material from which theprinthead is made: gain-cluster pull-out occurs to a greater or lesserextent during formation of the channel, leaving walls having microscopiccrevices, undercuts and overhangs.

These problems may be understood more clearly by reference to FIG. 4which is a very much enlarged view of a channel 112 defined by walls 116and 116a. The coating of the surface 150 of the wall 116 usingconventional line-of-sight deposition procedures such as ionimplantation or ion plating, which require line of sight 152 between thecoating source and the surface to be coated, is not possible. It islikewise impossible to coat undercut zones such as 154, 156 and 158 eventhough they are not shadowed by the opposite wall 116a of the channel.Consideration of the geometry will also show that this second problembecomes even more acute with increase in the aspect ratio of thechannel; that is, the ratio of the depth of the channel to its width:the greater the aspect ratio, the more acute is the maximum possibleangle between (a) the line of sight between the source and the channelbottom and (b) the plane of the channel wall, and thus the greater thearea of undercut that is placed in shadow by the overhang above. Suchacute angles also present problems in achieving a uniform and continuouscoating over the walls and channel bottom since the capture efficiencyof coating materials varies with the angle of incidence.

Yet another problem is caused by the manner of construction of ceramicpiezoelectric ink jet printheads of the type mentioned at the beginningof the description: the configuration of the electrodes, which arearranged so as to generate an electric field perpendicular to thedirection of polarisation of piezoelectric material thereby to displacethe piezoelectric wall actuators in shear mode makes it very difficultif not impossible to perform any subsequent repoling of thepiezoelectric material once passivation has taken place. Furthermore,ink jet print heads of the type in question are preferably made from ahigh activity piezoelectric ceramic having a Curie temperature (i.e. thetemperature T_(c) at which the material is no longer capable ofretaining polarisation) of the order of 150° C. to 250° C. The coatingprocess should be performed at a lower temperature, suitably 50° C. to100° C. below the Curie temperature, to avoid accelerated aging ordepoling of the piezoelectric material. The use of conventional chemicalvapour deposition or plasma-enhanced chemical vapour deposition coatingprocedures, which generally employ temperatures substantially in excessof 200° C., e.g. 300° C. or 500° C. or even more, therefore necessitatesrepolarisation following passivation if printhead activity (and henceefficiency) is not to be lost. To avoid repolarisation followingpassivation, a coating process temperature of less than 200° C., andpreferably not more than 100° C., is required, the lower temperaturespermitting the use of more active materials.

At lower temperatures, either coating is not achievable at all withthese procedures or it is only achievable at an unacceptably slow rateand in any event there is another problem which is that coatingthicknesses tend to decrease from top to bottom of the channel and thusunder the conditions required to achieve the desired thickness ofcoating towards the bottom of the channels, the deposition of coatingsof excessive thickness at the top is unavoidable, and as the tendencyfor the coatings to have defects such as unrelieved internal stressesincreases with thickness, the risk of obtaining a defective coating isincreased. This problem becomes particularly acute where the channel hasan aspect ratio of more than 2:1, e.g. 3:1 or more. For example, forsome channels where the aspect ratio is 4:1 or more, coating thicknessesof as much as one half or one micron may be found in the upper parts ofthe channel under the conditions required to achieve a desired coatingthickness of 50-100 nm lower down. Channels having an aspect ratio of3:1 or more are hereafter referred to as deep channels.

The present invention aims to solve the above problems.

According to the present invention, there is provided a process for thepassivation of the channel walls of a deep channel ink jet print headchannel of ceramic piezoelectric material by the deposition of a coatingcomprising inorganic material, the process comprising:

(a) providing an ink jet print head component containing said channeland

(b) while maintaining the bulk temperature of the actuating componentwhich contains said channel at a temperature of below 200° C. and atwhich not more than 30% depolarisation of the material occurs duringpassivation, exposing the surface of the channel walls to be passivatedto a homogenised vapour of the coating material, said vapour havingundergone multiple scattering during transport thereof from the sourceof the vapour to said surface.

By a homogenised vapour, we mean that the chemical constituents of thevapour used by the process have a substantially uniform distribution, sothat the coating deposited approaches and preferably attains chemicalhomogeneity in the surface layer.

By multiple scattering, we mean at least 2 and preferably at least 3scattering events. The vapour atoms are then substantially homogenisedin the sense that the energy and incident angle of the vapour atoms onthe surface is substantially randomised. If less than one collision(scattering event) occurs, the process is substantially line of sightwhereas if more than 3 collisions occur only a small fraction of atomsarrive directly from the source. On the other hand, if the number ofscattering events is too high, the vapour is in effect thermalised andthus it is preferred that the number of collisions does not exceed 8 or9 and more preferably does not exceed 6.

While kinetic modelling is in general too complex to simulate theinteractions that contribute to the required coating distribution andquality in deep channels, it is believed that some degree of surfacescattering of incident species from one side of the channel to the otherside of the channel takes place and this helps to equalise the coatingthickness from top to bottom of the channels. As the coating thicknessbuilds up, surface mobility and forward sputtering promoted by theincident flux, for example of ionised species under the bias field, alsocontribute to equalise the coating thickness and to covering hiddenfeatures in the surface. It is believed that the species is capable ofmigration on the surface due to surface mobility with a range typicallyexceeding 1 micrometer and that the range increases as the layer ofdensified coating develops. The range can be varied by changing both theproportion of ionised species in the vapour and their incident energy.Both transport of coating material down the surface of the wall in thechannel and spreading of material over the surface roughness featurestherefore occur during the process.

While the process is applicable to any deep channel ink jet print headit finds particular application to multi-channel print head channelsparticularly those containing electrodes and especially those whereinthe actuating component containing the channel is polarised in adirection substantially parallel to the planes of the channel walls, asin, for example, print heads having actuators of the so-called chevronor cantilever type.

In one preferred embodiment, which assists in reducing the risk ofincluding defects such as unrelieved internal stresses, the coating isformed by depositing a plurality of layers. These layers may bedeposited from vapours having the same composition, which assistsretaining chemical homogeneity of the coating throughout its thicknessor, as discussed in more detail below, they may be derived from vapoursof differing chemical compositions or from a vapour whose chemicalcomposition is varied during the period of deposition of the coating.

An acceptable coating rate while avoiding induced stress is achieved byoperation at high pressure, for example a pressure up to 200 mtorr(millitorr) but preferably not lower than 0.1 mtorr. If a pressure above200 mtorr is used the atoms arrive at the surface having lost too muchenergy and the material quality is therefore poor. On the other hand, ifthe pressure is less than 0.1 mtorr, the number of scattering events inthe vapour during transport from the source to the surface may becomeinadequate and the process may become "line of sight". A preferred rangeis 1 to 50 mtorr and the choice of pressure will depend inter alia onthe distance between the source and the substrate, the nature of theprocess gases and the temperature of the vapour.

Examples of suitable deposition methods are chemically reactivedeposition methods wherein the surface mobility of the layer-formingspecies is raised above the level predicated by the surface temperature;that is to say, methods which raise the surface mobility of thelayer-forming species by non-thermal means. Particular examples of suchmethods include electron cyclotron resonance (ECR)-assisted CVD e.g. asdescribed in J. Applied Physics 66, No 6, pages 2475-2480, and reactiveunbalanced magnetron sputtering (UMS) such as described in J. VacuumSciences Technology 4, No 3, pages 452 on. No applied heat is requiredwith these techniques and thus the risk of depoling and/or ageing thepiezoelectric ceramic material is minimised. Also by means of thesemethods, a continuous coating can be obtained even in those areas shadedfrom the sources of the layer-forming species e.g. due to overhang orsurface roughness. Another suitable process is UV photon assisted CVD. Afurther involves the use of organometallic precursors in a CVD process.

While not essential to the process, it has been found advantageous toapply a bias voltage. It has been observed for example, that this mayincrease the rate of deposition and/or the rate of deposition on thelower parts of the side walls of the channels relative to the upperparts and/or may improve the quality of the deposited layer, e.g. itsphysical and/or electrical properties. Good results have been obtainedat bias voltages of up to -300 v (target against ground) and even highervoltages may be found suitable in some cases. However, other conditionssuch as current level, should be chosen to avoid problems such assputtering of the layer being deposited and/or damage of the PZT byinduced heating. It will also be understood that there may be arelationship between the operating temperature and the bias voltage inthat the use of higher bias voltages may require a reduction in the bulktemperature of the actuator to avoid inadvertent depoling, and viceversa.

It has been found that the optimum bias may vary with the nature of thelayer being deposited and thus the passivation of the wall of apiezoelectric ceramic ink jet print head channel by building up thedesired coating thickness by depositing a plurality of layers bychemically reactive deposition, or other method involving chargedspecies, may be enhanced by the application of a bias voltage andvarying the level of bias voltage according to the nature of the layere.g. to minimise the level of stress in each of the deposited layers.

In order to achieve a strong bond between the coating and the substrate,it is desirable for the vapour to which the surfaces to be coated areexposed to have an energy at the surface of at least 1 eV if a surfacecatalytic effect is present or at least 5 eV if there is no catalyticeffect. At these energy levels, chemical bonding is encouraged whereasat lower levels, the bonding will be mainly physical. However, at highenergy levels, the substrate and/or the coating may be damaged and it istherefore not advisable for energy levels to exceed 500 eV, andpreferably they are below 300 eV and more preferably below 100 eV.Whereas a range of 5 to 25 eV, and more particularly 12 to 20 eV, isexpected to be appropriate for most circumstances to develop a densecoating layer, higher energies, depending on the vapour are useful topromote transport and spreading of the layer-forming species.

Two or more than two layers may be deposited by the process of theinvention and the layers may be of the same composition; however aparticular advantage of the process is that layers of differentcomposition may be deposited. The thickness of the various layers mayalso be varied, thereby providing the operator with a very versatiletool for achieving particular properties and combinations of propertiesin the coating, e.g. in terms of resistivity, ion barrier properties andwater permeability. One particular advantage arises from the observationthat the rate at which a layer is deposited depends on its composition.Thus, the rate at which a coating with a particular overall thicknessand particular properties is obtained can be increased by firstdepositing a layer having a higher rate of deposition followed by afurther layer having the composition having the desired properties.

Any material capable of being deposited by the process of the inventionmay be employed in the formation of the layers making up the passivationmultilayer coating. The material may comprise an element, e.g. as incarbon or a metal, or it may be a combination of two or more elements asin a metal alloy or a compound. (By a "compound" we mean here acombination of two or more elements whether in the ratios dictated bytheir valencies or not). This is because where a compound is depositedit has been found that the ratios of the elements in the deposited layermay be varied from those strictly expected from their respectivevalencies and that these ratios can be controlled by control of theprocess conditions in known manner. Thus, for example, a layer ofsilicon and carbon may be deposited wherein the ratio of Si to C isother than 1:1; moreover, the ratio may be varied, if desired, as thelayer is deposited.

Examples of layers that may be deposited include carbon (both amorphousand diamond-like), silicon-oxygen(SiO), silicon-nitrogen(SiN),silicon-oxygen-nitrogen(SiON), silicon-carbon (SiC), aluminium-nitrogen(AlN), silicon-aluminium-nitrogen (SiAlN), aluminum-oxygen (AlO),aluminium-silicon-oxygen (AlSiO) and silicon-aluminium (SiAl).

It is to be understood that the symbolic representations of elementalcombinations given in brackets in the preceding sentence are notintended to be indicative of any specific stoichiometric ratios, andthat the deposited layers may comprise stoichiometric, nonstoichiometricand/or doped combinations of the indicated elements. For example a layerreferred to as an SiO layer may contain Si and O atoms in a ratio of 1:2or in different ratio and a layer referred to as an SiN layer maycontain Si and N atoms in a ratio of 3:4 or in a different ratio.

The materials employable as sources for the elements employed in theformation of such layers by CVD are known; for example silanes may beemployed as a source of silicon, hydrocarbons as a source of carbon, andammonia, and oxides of nitrogen, as well as nitrogen itself, as a sourceof nitrogen. H and/or O atoms from unavoidable water vapour impurity mayalso be included in the layers. For example, SiN layers may also containhydrogen and/or oxygen atoms. SiO layers may also be found to containnitrogen atoms.

Where the ink jet print head is intended for use with an ionisable inkit is desirable for the passivation layer to comprise both an electronbarrier and an ion barrier but it may be difficult to contrive a singlematerial that efficiently provides both these properties. Thus, apreferred multi-layer arrangement includes at least one electron barrierlayer and at least one ion barrier layer. Thus in one preferredembodiment there is provided by the process of the invention apassivated ceramic piezoelectric ink jet print head channel wherein thepassivation comprises at least one layer of material which provides anion barrier, preferably SiN, and at least one layer of material whichprovides an electron barrier, preferably SiO. Preferably a layer ofelectron barrier material is located between the channel wall and alayer of ion barrier material.

In general, it will be desirable for the electron barrier layer to havea resistivity of at least 10¹³ ohm.cm and for the ion barrier layer topass an ion current not greater than 1 nA/cm² at an applied field of10V/micron. It is also generally preferable that the ion barrier layerdoes not break down under fields of less than 10V/micron and morepreferably 30V/micron.

In another preferred embodiment, the passivation multilayer includes thelayer structure SiO/SiN/SiO(SiN/SiO)_(x) where x is zero or a positiveinteger, and with the first SiO layer nearest the channel wall.

In a further preferred embodiment, the passivation multilayer mayinclude a conducting layer electrically insulated from the channel wall(or more particularly from the electrodes associated with the channel)by another layer of the multilayer. Such a conducting layer may providethe effect of a Faraday's cage the presence of which is advantageoussince it enables ink in the channel to be protected from electric fieldsemanating from the channel electrodes. This is particularly importantwhere the ink is a dispersion.

It also assists in confining to the channel sidewalls stray electricfields emanating from the electrodes, thereby reducing the risk ofpiezoelectric cross-talk between channels in a multi-channel array.

Thus, according to yet a further aspect of the invention there isprovided a ceramic piezoelectric ink jet print head channel the walls ofwhich are passivated and the passivation includes a conducting layerelectrically insulated from the channel walls by another layer andproviding a Faraday's cage effect.

In a preferred embodiment of this aspect of the invention, theconducting layer is provided in a multilayer arrangement between thechannel wall (and in particular the electrodes associated with thechannel) and a layer of ion barrier material. By means of thisembodiment this layer of ion barrier material is protected from theelectromagnetic fields emanating from the channel electrodes.

A particularly preferred embodiment of this aspect of the inventioncomprises a passivation multilayer comprising at least one ion barrierlayer, at least one electron barrier layer and a conducting layer, withan electron ion barrier layer (i.e. insulation) located between thechannel wall (electrode) and the conducting layer and an ion barrierlayer on the other side of the conducting layer; i.e. between theconducting layer and the ink.

Alternatively, if the conducting layer insulated from the channelelectrodes is in contact with the ink, it may be used to control thepotential of the ink independently of the electrode potential duringactuation. This may assist to control the charge carried by ink dropsejected from the print head as described in British patent application93:22203.2.

Any suitable material may be employed for the conducting layer and whileit is advantageous, from the point of view of simplifying the equipmentemployed to produce the passivation multilayer, for the material to besuch that the layer is obtainable by CVD, this is not essential.Examples of suitable materials are metals, including alloys; howeverparticularly preferred are silicon carbide (SiC) and carbon since anapparatus designed to produce the preferred ion and electron barriermaterials of SiN and SiO may readily be adapted to produce layers of SiCand/or carbon e.g. using a hydrocarbon such as methane as the carbonsource.

Carbon is a particularly noteworthy material for one or more layers ofthe multilayer passivation since according to the deposition conditionsemployed it may be deposited either as an insulating layer (e.g.diamond-like carbon) or as a conducting layer (e.g. amorphous carbon).

Thus according to another preferred embodiment of this aspect of theinvention, a conducting layer of the passivation multilayer compriseselectrically conductive carbon, e.g. amorphous carbon, and preferablysuch passivation multilayer also includes an electrically insulatingcarbon layer, e.g. diamond-like carbon.

Another preferred embodiment comprises a passivation multilayerincluding an electrically insulating carbon layer, e.g. diamond-likecarbon and preferably also an electrically conducting carbon layer, e.g.of amorphous carbon.

In certain cases, e.g. in the presence of a water-based ink, it will bedesirable to include a water barrier layer. Suitable pinhole-free waterbarrier layers preferably include the materials aluminium oxide,diamond-like carbon and aluminium nitride but any of the materialslisted above may be suitable in the absence of an applied field. Themoisture permeation coefficient of the layer should be no more than10⁻¹³ gm.cm/cm² sec. cm H₂ as measured by the experimented procedurebased on ASTM E96-53T.

It will be understood that the passivation multilayer may also includeother layers than those specifically mentioned above. For example, itmay be desirable first to deposit on the channel wall an underlayer toassist adhesion of the remaining layers of the multilayer to the channelwall and/or the electrode material thereon. Similarly, where the printhead is intended for use with certain inks, it may be desirable todeposit, as the final layer, a material having specific chemicalresistance to prevent damage to the other layers by components of theink.

As indicated above, the composition of a layer may be varied as it isdeposited. Thus, for example, in the deposition of an SiN layer, theratio of Si:N may be altered during the course of the deposition.Likewise, for example, if an SiN layer is to be followed by an AlNlayer, the process may be controlled so that the ratio of Si:Al isvaried from 100:0 to 0:100, thereby giving an intermediate zonecontaining Si-Al-N between Si-N and Al-N. The variation of thecomposition may be continuous or stepwise.

The channel walls of the deep channel to which the process of thepresent invention may be applied may be of any piezoelectric ceramicmaterial. Examples include both crystalline ceramic materials such asgadolinium molybdate (GMO) and Rochelle salt, and polycrystallineceramic materials such as lead zirconate titanate (PZT) and relatedpiezoelectric perovskite ceramics. Specific examples include Motorola HD3203 (T_(c) =260° C.), Sumitomo HD5 (T_(c) =205° C.) or Tokin N-10(T_(c) =165° C.).

The invention is now illustrated by the following Examples which involvethe coating of a PZT ink jet print head channel having parallel sidewalls and a bottom wall, and a width of 90 μm and a depth of 500 μm.

In a first experiment to show the benefits of LTCRD, passivation wasdeposited with no applied heat, using an Astex ZX4400 ECR-CVD source. Achamber pressure of 1-5 millitorr, back filled with 5% silane in argonand nitrogen with no applied bias, was used to deposit a single nitridelayer. The distance between the source and the substrate was about 6 xthe mean free path of the vapour mixture. The thickness of the layer atthe bottom of the sidewalls was found to be 19% of the thickness on thetop horizontal surface, compared with only 14% using plasma enhancedCVD. In a second experiment with bias exceeding -50V, the bottom layerthickness increased to 28%. The thickness at the top of the walls ineach case is approximately 50%. Thus a desired minimum thickness can beachieved at the bottom of the sidewall with a lower thickness ofmaterial at the top of the sidewall. This not only reduces thelikelihood of stress in the layer, but also shortens the depositiontime. Moreover, the plasma enhanced CVD process required a temperatureof 300° C. which is substantially above the maximum tolerabletemperature for processing most PZT materials without the risk ofdepoling. Analysis of the material revealed a hydrogen content of lessthan 12 at %, and a buffered HF etch rate (7:1 dilution) of less than 25Ångstroms.min⁻¹. The coating exhibited excellent adhesion to the PZT, noexfoliation and no observed crack sites. The coating had a resistivityof greater than 10¹³ Ohms.cm at 10 KHz, a series resistance of about 10⁹Ohms, and a dielectric constant of 7 (at 1 MHz and 50 mV).

In a third experiment, a 1.1 μm thick passivation coating, (measured byERDA on the horizontal top surface) was formed using ECR-CVD apparatuswith an applied bias of up to -150V. The coating comprised a pluralityof layers as follows: (PZT)/SiO/SiN/SiO/SiN/(Air). The gases used toform the SiO layers were 5% silane in argon, and nitrous oxide. Thelayers were substantially SiO₂, with less than 10% atomic hydrogen. Thegases used to form the SiN layers were 5% silane in argon and nitrogen.The layers were substantially a-Si₃ N₄ :H, with less than 20% atomichydrogen. The coating had excellent adhesion to the PZT with no stresscracking, and was not removed by the Sellotape test.

In similar fashion, multilayer passivation coatings of the followingstructure could be obtained.

(PZT)/SiO/SiC/SiN/(Air)

(PZT)/SiO/amorphous carbon/(Air)

(PZT)/diamond-like carbon/amorphous-like carbon/(Air)

The SiO layers, which were substantially SiO₂, with less than 10% atomichydrogen, were derived as described above. The SiC layer was derivedfrom 5% silane in argon and methane. The SiN layer, which wassubstantially a-Si₃ N₄ :H with less than 20% atomic hydrogen was derivedas described above. The amorphous and diamond-like carbon layers wereobtained using methane and argon.

We claim:
 1. A process of passivating the channel walls of a deepchannel ink jet print head channel of ceramic piezoelectric material,the process comprising the steps of:(a) providing a deep channel ink jetprint head component containing a channel of ceramic piezoelectricmaterial having channel walls; and, (b) while maintaining the bulktemperature of the component which contains said channel at atemperature of below 200° C. and at which not more than 30%depolarisation of the ceramic piezoelectric material occurs duringpassivation, and while maintaining an operating pressure of at least onemillitorr, exposing a surface of the channel walls to be passivated to ahomogenised vapor of a coating material comprising inorganic material,said vapor having undergone multiple scattering during transport thereoffrom a source of the vapor to said surface and striking the surface. 2.A process as claimed in claim 1 in which the vapour undergoes from 2 to9 scattering events during transport thereof from the source to thesurface.
 3. A process as claimed in claim 1 wherein vapour undergoes 3to 6 scattering events during transport thereof from the source to thesurface.
 4. A process as claimed in claim 1 in which the print headchannel includes electrodes.
 5. A process as claimed in claim 1 whereinthe actuating component comprises piezoelectric ceramic operating inshear mode.
 6. A process as claimed in claim 5 in which the actuatingcomponent containing the channel is polarised in a directionsubstantially parallel to the planes of the channel walls.
 7. A processas claimed in claim 6 in which the actuating component is of the chevronactuator or cantilever actuator type.
 8. A process as claimed in claim 1wherein the coating is formed of a plurality of layers.
 9. A process asclaimed in claim 1 wherein said vapour has an energy of at least 5 eV atthe surface.
 10. A process as claimed in claim 9 wherein the energy ofsaid vapour at the surface is in the range 5 eV to 25 eV.
 11. A processas claimed in claim 9 wherein the energy of said vapour at the surfaceis in the range 12 eV to 20 eV.
 12. A process as claimed in claim 1wherein the energy of said vapour at the surface is not greater than 100eV.
 13. A process as claimed in claim 1 wherein the energy of saidvapour at the surface is not greater than 500 eV.
 14. A process asclaimed in claim 1 wherein the energy of said vapour at the surface isnot greater than 300 eV.
 15. A process as claimed in claim 1 which isoperated at a pressure of not greater than 200 millitorr.
 16. A processas claimed in claim 1 which is operated at a pressure in the range of 1to 50 millitorr.
 17. A process as claimed in claim 1 wherein the coatingis effected by a chemically reactive deposition method wherein thesurface mobility of the layer-forming species is raised above the levelpredicated by the temperature of the surface being coated.
 18. A processas claimed in claim 1 wherein the coating is effected by electroncyclotron-assisted chemical vapour deposition, reactive unbalancedmagnetron sputtering or UV photon assisted chemical vapour deposition.19. A process as claimed in claim 1 which employs organometallicprecursors in a chemical vapour deposition process.
 20. A process asclaimed in claim 1 in which a bias voltage is applied.
 21. A process asclaimed in any claim 1 wherein the passivation comprises deposition ofat least one of an ion barrier layer, an electron-barrier layer, aconductive layer and a water-impermeable layer.
 22. A process as claimedin claim 1 in which the coating comprises one or more layers eachselected from carbon, silicon-carbon, silicon-nitrogen, silicon-oxygen,silicon-oxygen-nitrogen, silicon-aluminium, silicon-nitrogen-aluminium,aluminium-oxygen and aluminium-silicon-oxygen.
 23. A process as claimedin claim 1 in which a plurality of layers of differing compositions aredeposited.
 24. A process as claimed in claim 23 which comprisesdepositing an electron barrier layer and an ion barrier layer.
 25. Aprocess as claimed in claim 24 wherein the electron barrier layer isbetween the channel wall and the ion barrier layer.
 26. A process asclaimed in claim 24 wherein the material of the electron barrier layeris selected from silicon-oxygen and diamond-like carbon.
 27. A processas claimed in claim 24 wherein the ion barrier layer comprisessilicon-nitrogen.
 28. A process as claimed in claim 23 which comprisesdepositing an electron barrier layer followed by an electricallyconducting layer.
 29. A process as claimed in claim 28 which furthercomprises depositing an ion barrier layer over the electricallyconducting layer.
 30. A process as claimed in claim 28 wherein thematerial of the electrically conducting layer is selected from amorphouscarbon and silicon-carbon.
 31. A process as claimed in claim 28 whereinthe material of the electron barrier layer is selected fromsilicon-oxygen and diamond-like carbon.
 32. A process as claimed inclaim 28 wherein the ion barrier layer comprises silicon-nitrogen.
 33. Aprocess as claimed in claim 23 in which said plurality of layersincludes a conducting layer electrically insulated from said channelwall by a further layer to provide a Faraday's cage effect.